1. Field of the Invention
The present invention relates to frequency hopping wireless systems, and, more particularly, to synchronization of frequency hopping wireless systems.
2. Description of the Related Art
According to FCC regulations, any short-range wireless system that operates in frequency bands 902-928 MHz or 2400-2483.4 MHz should employ either frequency hopping methods or use digitally modulated techniques. Similar requirements are present in European standards (EN300-220) regarding the use of frequency band 863-870 MHz.
In the frequency hopping methods, the system divides the available frequency bandwidth into many frequency channels, and it continuously switches the channel, using a pseudorandom frequency hop sequence known to both the transmitter and the receiver. The FCC regulation for the 902 MHz band requires the system to use a minimum of fifty frequency channels. The nodes in the network follow a pseudorandom frequency hop pattern as the system switches between different frequency channels. The period during which the system stays in one frequency channel before hopping to the next channel is referred to as “dwell time”. The FCC and EN300-220 regulations specify a maximum dwell time of 400 milliseconds. The dwell time can be fixed or it can change for different transmissions. However, each transmitter should, on average, use each channel in the frequency hop sequence equally. In addition, the FCC regulation specifies a maximum channel usage by the system: The average time of occupancy on any frequency channel should not be greater than 400 milliseconds within any window having a length of twenty seconds.
Although both the sender and the receiver (and in general all the nodes in the system) are assumed to know the frequency hop sequence, in order to have a successful communication they should also be in synchronization in the sense that they should both know which position in the frequency hop sequence is being used at any moment in time. The system should provide a mechanism for the newly added nodes, or existing nodes that have lost the time-synchronization, to get in synchronization with the rest of the network and find the current frequency hop index in the frequency hop sequence.
Synchronization latency is an important design factor. A newly added node should be able to get in synchronization with minimal delay to start the communication with the remainder of the network. The acceptable latencies for the synchronization process in many applications are on the order of one to two seconds.
On the other hand, it is desired to use as few transmissions as possible for the synchronization process. This is important because the regulation imposes a limit on the period of time that each channel can be used by the system (i.e., 400 milliseconds in any window of twenty seconds). The less transmission time that is used for the synchronization process, the more time remains for other network operations.
What is neither disclosed nor suggested by the prior art is a synchronization method that enables fast synchronization while, on average, the transmission time on each channel is reduced.